SyNCHROTRON-BASED NEAR-EDgE X-RAy SPECTROSCOPy discrete radii indexed by a principal quantum number n = 1,2,3 …, and discrete binding energies for single-electron atoms of E n = −E 0 Z 2 /n 2 , where Z is the atomic number and E 0 = 13.6 eV (see Figure 17.1). When the energy of incident photons is increased to match the binding energy of an electron, the photon absorption probability suddenly increases in what is called an X-ray absorption edge (step function in Figure 17.2, which is at about 290 eV for carbon), so that the electron is completely removed from the atom in an ionization event (absorption edges are also labeled figure 17.1. Atom in the Bohr model. The electrons surround the nucleus. An incoming photon can lift an electron to a not fully occupied, higher orbital or remove it entirely from the atom.
Energy (eV)280 285 290 295 300
Normalized absorption
Measured ModeledStep functionfigure 17.2. Carbon NEXAFS spectrum of NOM from the Suwannee River (IHSS standard humic acid mounted on indium foil; total electron yield using a dwell time of 200 msec and an exit slit of 50 µm, calibrated to CO at 287.38 eV, Canadian Light Source SgM beamline 11-ID.1) to show pre-edge features and the so-called "edge". The spectrum is deconvoluted using a series of gaussian curves (g) at energy positions of known transitions, along with a step function at the edge as described by .